This invention relates to a method for annealing electrodeposition structures formed by electrodeposition techniques particularly suitable for use in electroforming.
U.S. Pat. No. 4,623,503 to Anestis et al. entitled xe2x80x9cSlush Molding Method With Selective Heating of Mold By Air Jetsxe2x80x9d, assigned to the assignee of the present invention and hereby incorporated by reference, discloses a method of slush molding with the use of an electroformed nickel mold.
According to U.S. Pat. No. 4,108,740 to Wearmouth entitled xe2x80x9cHard, Heat-Resistant Nickel Electrodepositsxe2x80x9d, the production of electroforms involves building up deposits of adequate thickness on a mandrel without internal stress in the deposit so high as to cause premature separation of the deposit from the mandrel. The ""740 Patent goes on to state that the electroformability and hardness of nickel can be improved by electrodepositing the nickel from an electrolyte containing addition agents which introduce sulfur into the resulting electrodeposit and that, while sulfur improves electroformability by reducing the internal stress in the electrodeposit, it does so at the expense of ductility. In the ""740 Patent, for example, it is reported that sulfur contents in excess of approximately 0.005% cause the electrodeposit to embrittle upon exposure to temperatures above about 200 degrees Celsius, and that embrittlement at temperatures above ambient is particularly disadvantageous in electroforms requiring exposure to elevated temperatures, in applications such as molds and dies, or in fabrication such as screen printing cylinders which can be subjected to localized heating by brazing, welding or by the use of heat curable glues, or during surface masking using heat curable lacquers.
According to U.S. Pat. No. 5,470,651 to Milinkovic et al. entitled xe2x80x9cMandrel For Use in Nickel Vapor Deposition Processes And Nickel Molds Made Therefromxe2x80x9d one drawback of electroformed nickel shells and molds, in consequence of the fact that electroformed nickel contains relatively large amounts of sulpher, is that repairs or modifications to the shell or mold by means of welding cannot be preformed readily.
In addition to the above drawbacks, the Applicant has found that electrodeposition structures, such as the electroformed molds discussed above, may contain voids within the electrodeposition structure itself. These voids are formed during the build-up of deposits on the mandrel and are ordinarily of microscopic size, generally round in shape and on the magnitude of less than 0.005xe2x80x3 in size.
Applicant has also found that, during heating of the electrodeposition structure, these voids, depending on their proximity to the surface of the electrodeposition structure, may cause the surface of the electrodeposition structure to distort in the form of a protuberance, similar to that of a bulge or bump, on the electrodeposition surface. Without being bound to a particular theory, the Applicant believes that heating of the electrodeposition structure causes pressure from gas, believed to comprise hydrogen generated and entrained during formation of the electrodeposition structure, within the void to increase. As a result, particularly in those areas of the electrodeposition structure where the voids are nearest the surface, the increase in gas pressure within the void overcomes the bending strength of the thin electrodeposition thickness above the void and forces the surface of the electrodeposition structure to rise.
In those instances where the voids produce surface protuberances, the Applicant has found that the voids may be repaired via welding. However, more problematic is whether the texture of the surface of the weld and surrounding electrodeposition structure are uniform and blended as to completely hide the presence of the repair. Applicant has found that the ability to repair the surface of the weld and surrounding electrodeposition structure adequately depends largely on the texture of the surface of the electrodeposition structure. Many of the electroformed molds used in the automotive industry have a grain texture formed on the electrodeposition surface. In some instances the texture of the electrodeposition surface can be repaired, while in other instances it cannot be successfilly repaired as the grain pattern cannot be replicated in the repaired areand Thus, at the very least, voids in the electrodeposition structure result in costly repairs and time and, on occasion, the complete electrodeposition structure becomes scrap.
Furthermore, Applicant believes that while certain of the voids contained within the electrodeposition structure may not produce protuberances on the surface of the electrodeposition structure in response to heating of the structure, nevertheless Applicant believes these voids may weaken the overall electrodeposition structure resulting in premature cracks, metal fatigue, etc. of the electrodeposition structure.
What is needed is a process to anneal an electrodeposition structure to make the structure more ductile so as make the structure more receptive to repairs or modifications by means of welding. What is also needed is a process to anneal the electrodeposition structure such that the likelihood of voids which may be formed in the structure, giving rise to protuberances on the surface of the structure during heating, is reduced and more preferably eliminated.
Accordingly, one of the objects of the present invention is to provide a new and improved process for providing electrodeposition structures that have improved grain structure and reduced voids which may cause surface disruption.
Another object of the present invention is to provide an electrodeposition structure having greater ductility and a reduced propensity for surface disruption.
A further object of the present invention is to provide an annealing process that provides electrodepositon structures that are easier to repair.
The above objects and others are realized in accordance with the invention by a method in which an electrodeposition structure is exposed to heat and pressure above ambient to increase the ductility and change the grain structure of the electrodeposit. In one form of the invention, the electrodeposition structure is exposed to and held at a temperature between and including 48 and 99% of the melting temperature of the electrodeposit in an argon gas atmosphere. Upon cooling to ambient, improvements in ductility and grain structure of the electrodeposit were noted.
In another form of the present invention, an electrodeposition structure is heated to and held at a temperature between and including 48 and 99% of the melting temperature of the electrodeposit under argon gas at 15,000 psi. Upon returning the structure to ambient conditions, further improvements in ductility and grain structure were noted.
In another form of the invention, a method for annealing a structure formed by electrodeposition is disclosed, the method comprising first providing the electrodcposition structure, the electrodeposition structure comprising an electroformed mold, the electroformed mold having a nominal thickness between and including 0.5 mm to 8.0 mm and having a melting temperature; heating the electrodeposition structure to a temperature between ambient temperature and the melting temperature of the electrodeposition structure; isostatically pressurizing the electrodeposition structure to a pressure above ambient pressure; cooling the electrodeposition structure to ambient temperature; and depressurizing the electrodeposition structure to ambient pressure.
In yet another form of the invention, an electroformed mold is disclosed, the electroformed mold annealed at an annealing temperature above ambient temperature and an annealing pressure above ambient pressure wherein the electroformed mold comprises a material having an elongation measured at break before and after annealing, the elongation at break after annealing being greater than the elongation at break before annealing.
In yet another form of the invention, an electroformed mold is disclosed, the electroformed mold comprising a material having voids therein, at least a portion of the voids forming at least one protuberance on the surface of the electroformed mold when the mold is exposed to heat wherein the electroformed mold is annealed at an annealing temperature above ambient temperature and an annealing pressure above ambient pressure and wherein the number of voids forming protuberances on the surface of the electroformed mold is reduced after annealing of the electroformed mold as compared to before annealing of the electroformed mold.